The present invention relates to a method and apparatus using ion chromatography ("IC") followed by chemical conversion and detection of the sample ions.
Ion chromatography is a known technique for the analysis of sample in an eluent solution containing an electrolyte. The sample solution is injected into a chromatographic separation zone, in the form of an ion exchange column, and directed through an eluent suppression stage, and a detector, typically a conductivity detector. Ions of the injected sample are separated on and eluted from a separation column. In the suppression stage, electrical conductivity of the eluent electrolyte, but not that of the separated ions, is suppressed. This can be accomplished so long as the separated ions are not derived from very weak acids or bases and so can be determined by conductivity detection. This general technique is described in U.S. Draft Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,956,559. The above patents, incorporated herein by reference, describe suppression or stripping of electrolyte using an ion exchange resin bed device (called "a packed bed suppressor" or "PBS"). This general type of suppressor requires periodic shut-down for regeneration and performs in a batch rather than a continuous mode.
Disadvantages of the packed bed suppressor approach as it has been practiced are well documented. Such suppressors normally were run until the end of a shift or until exhaustion of the ion exchange resin, if earlier, followed by regeneration of the PBS. The PBS included a large volume of high capacity ion exchange resin. A typical ratio of the volume of the ion exchange resin in the suppressor to that of the separator column ranged from about 2.0 to 0.5 to one, thereby suppressing the developing reagents from a large number of separation runs (e.g. 15 to 50) prior to regeneration. Similarly, under such conditions, the ratio of capacity of the suppressor to that of the separation column was on the order of 100 to 700 to one.
Some disadvantages of the PBS approach are set forth in U.S. Pat. No. 4,474,604. For example, it limits the number of samples which can be consecutively analyzed. (Col. 1, lines 38-41). Also, certain difficulties are caused by the variable length of non-depleted resin in the column as it is being used up. This factor can vary elution times of certain ions, with less or no effect on other ions. (Col. 1, lines 50-55).
Other disadvantages of using a packed bed suppressor in the manner of the prior art include (1) restriction of the number of sample injections by the capacity of the suppressor, and (2) extra band spreading in the suppressor column resulting in lower resolution. (J. of Chromatog., 1981, 218, 57, at 58). This is because the separated ionic species are re-mixed in the volume of the suppressor, resulting in a loss of resolution (peak broadening). The suppressor volume is thus a compromise between regeneration frequency and chromatographic resolution. Because of interaction with the suppressor column, the peak height of nitrite ion dramatically changes as a function of suppressor exhaustion. (J. of Chromatog., 1982, 237, at 297).
An improved form of suppressor, called "a membrane suppressor", was developed to overcome these disadvantages. Significantly, the membrane suppressor is continuously regenerated during use, leading to its substantially replacing the packed bed suppressor. In a membrane suppressor, a charged membrane, normally in the form of a fiber or sheet, is used in place of the resin bed. In sheet form, the sample and eluent are passed on one side of the sheet with a flowing regenerant on the other side of the sheet. The sheet comprises an ion exchange membrane partitioning the regenerant from the effluent of chromatographic separation. The membrane passed ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to a weakly ionized form, followed by detection of the ions. One highly effective form of suppressor is described in U.S. Pat. No. 4,999,098.
The membrane suppressor minimizes many of the foregoing disadvantages. However, membrane suppressors also have certain disadvantages compared to packed bed suppressors such as cost, leakage of regenerant causing higher detector background, the requirement for an external supply of regenerant solution and the fact that membrane suppressors are complex and not user serviceable. Thus, it would be advantageous to develop an inexpensive, high performance packed bed suppressor approach.
Gradient elution is performed by changing from a weak to a strong eluent during a chromatography run. Gradient elution has been attempted for the PBS approach. However, such attempts were "less than successful". (Ion Chromatography, Small, Hamish (Plenum Press, 1989, p.187) An attempt to solve one of the problems of such systems is set forth in Suden, T., et al. Anal. Chem. 1984, 56, 1085. Gradient elution also has been used in a membrane suppressor IC system. (See, e.g. Rocklin, R. D., et al. J. Chromatogr. 411 (1987) 107.
Gradient elution is particularly useful for analytes of interest having widely different affinities for the chromatographic stationary phase. An example in ion chromatography might be the separation of fluoride and citrate using anion exchange. In this case, fluoride has low affinity for the anion exchange column while citrate on the other hand has high affinity. In order to resolve these components in a single chromatographic analysis, gradient elution is used. In gradient elution, the elution process begins with an eluent of low displacing power than increases over time to an eluent of greater displacing power. This can be accomplished by changing the concentration and/or composition of the eluent. While gradient elution solves a variety of separation problems, detection can be a problem since the detector typically is sensing some property of the eluent. Suppression converts the eluent to a low conductivity form so that the conductivity detector in IC senses only very small changes in the background conductivity during the gradient elution process.